chore: bump ember/render-modifiers to 3.0.0

[Techn1x]

Hello! This is to help this repo work with ember-source 6 which I've been getting dependency warnings for.

ember/render-modifiers 3 widens its peer dependencies to ember 6 https://github.com/emberjs/ember-render-modifiers/releases

I noticed this was merged recently (but unpublished) https://github.com/yapplabs/ember-tether/pull/130

After this is merged, can we get a release published? Thanks!